Abundance of encapsulated C 60 in single-wall carbon nanotubes

Spherical objects (presumably C ₆₀ molecules) have been observed inside ∼14 Å diameter single-wall carbon nanotubes. We applied acid etching to access the interior of the tubes, and leaching in toluene to dissolve liberated C ₆₀. Quantitative UV-Vis of the 335 nm C ₆₀ transition establishes beyond doubt that these are indeed C ₆₀, and that their relative abundance in a macroscopic sample can be as high as 5.4% (expressed as the filled fraction of total tube length). Variations in abundance with growth and processing conditions provide clues as to how they form.